This is a renewal of a previous grant that focused on hereditary hemorrhagic telangiectasia (HHT). The original grant triggered discoveries that linked the pathogenesis of HHT to mechanisms of arterial-venous identify and defined new roles for guidance cues in neurovascular regeneration. Here, we will use the genetics of cerebral cavernous malformations (CCM) to piece together a novel endothelial signaling pathway critical for vascular development and the pathogenesis of CCM. Loss-of-function mutations in three genes, KRIT1 (CCM1), OSM (CCM2), and PDCD10 (CCM3) cause CCM. In preliminary studies, we have made considerable advances in understanding the basic biology surrounding KRIT1 and OSM. These insights lead us to speculate that PDCD10 participates in a signaling pathway that is critical for endothelial cell architecture and cell-cell interaction. Our model would suggest that when both copies of PDCD10 are ablated, endothelial cells are severely compromised and fail to initiate the first angiogenic event in development, formation of the branchial arch arteries. When only one copy of PDCD10 is lost, as is the case in human disease, endothelial tubes form, however, the integrity of their barrier in response to stress is compromised, resulting in vascular leak. By gaining insights into the basic biological pathways regulated by PDCD10, we hope to point the way to future therapies for CCM.